Method of continuously annealing steel strip



April 29, 1958 F. J. KRAHE ETAL 2,332,711

METHOD OF CONTINUOUSLY ANNEALING STEEL STRIP Filed Jan. 16, "1956 2 Sheets-Sheet 1 .FR4A/c/s .1. KRAHE and BERNARD M. NEG/(RICH,

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METHOD F CONTINUOUSLY ANNEALING STEEL STRIP Francis J. Krahe, Tarentum, and Bernard M. Necltrich,

Pittsburgh, Pa., assignors to United States Steel Corporation, a corporation of New Jersey Application January 16, 1956, Serial No. 559,301

3 Claims. (Cl. 14821.5)

This invention relates to a method for continuously annealing cold-reduced strip steel, particularly low carbon steel strip.

Flat-rolled low carbon, i. e., containing not more than .2% carbon, steel represents approximately 50% of the rolled steel produced. By far the largest portion of this product is reduced to final gauge by cold rolling; gaugereductions as high as 85% being made in the latter operation. The efiect of such drastic cold reduction is to elongate and mash the microstructure of the steel, rendering it hard and unformable and necessitating a subsequent heat treatment to restore the desired degree of softness and ductility. The aim of the treatment is to produce sheet or strip to a specified hardness value suitable for its ultimate use.

Heretofore the heat treatment has generally consisted of box annealing the cold-reduced strip. In this operation several coils are stacked in a suitable enclosure and slowly heated to about the lower transformation temperature of the steel, soaked at this temperature for several hours and then very slowly cooled to a temperature of about 250 F., the entire cycle requiring 60 to 70 hours. This treatment, when properly conducted, results in a microstructure of equi-axed ferrite grains characterized by distribution of the carbides as small wellrounded particles. Such structure is considered by many to provide optimum combination of softness and ductility and to be the best for severe forming operations. Box annealing, however, is a very time consuming operation and in recent years there has been an increased shift to the use of continuous annealing. In the latter operation, the steel is passed as a strand through a furnace comprising a heating and a cooling section. The heating section is designed to bring the steel strip to a temperature in the range of 1450 to 1750 F. in /2 to 2 minutes and hold it in this range for several seconds; the cooling section is several hundred feet long and is arranged to counterflow cooled furnace atmosphere to bring the strip to the desired end temperature in a few minutes. Continuous annealing is well adapted to in-line-production of coated steel products since the strip can be cooled to a temperature just above the temperature of the coating metal and introduced immediately therein, thus effecting a considerable conservation of heat. Practice representative of conventional continuous annealing i depicted by curve 1 of Figure 1 of the attached drawings. In this type of operation, the heating section is operated with maximum heat input, and the strip speed regulated to achieve the specified hardness; while cooling is controlled only to the extent necessary for reducing the strip to a suitable temperature for introducing it into the atmosphere or entry into a molten metal coating pot. While conventional continuous annealing practice is directed toward obtaining a fully annealed product, i. e., the steel strip is heated to temperatures approaching or above its upper critical, the operation will not produce as soft a product as the older box annealing even when the strip atent O M 2,832,711 Patented Apr. 29, 1958 speed is decreased to provide additional time at temperature. Thus it has not been adapted to the processing of all grades of material, as most conventional practices will consistently reduce'the hardness of the cold-reduced low carbon strip to Rockwell B value of 58, and this result is achieved only by lowering strip speed, and decreasing production.

The annealing practice herein disclosed is aimed at consistently producing soft strip, i.v e. Rockwell B-55 maximum hardness, in continuous annealing furnaces at high speeds. critical annealing temperature range used in conjunction with a specific cooling and holding cycle will consistently produce the desired properties.

Annealing at subcritical temperatures is possible since ferrite grains which have been distorted and highly stressed as a result of cold rolling, have the ability to recrystallize and reorient themselves into a normal structure at temperatures as low as 1000 F. However, the pearlite grains which have also been stressed and distorted by cold rolling remain unatfected at subcritical temperatures. Prior practices in subcritical annealing have relied on long time at annealing temperature coupled with slow cooling to govern the degree of hardness realized from the annealing operation. This practice is based on the premise that steel must be held at subcritical annealing temperatures for long periods of time to permit recrystallization of the ferrite and agglomeration of the carbides. The annealing practice of this invention is a radical departure from the recognized art in subcritical annealing as is apparent from the illustration of our improved practice as depicted by curve 2 of Figure l of the drawings.

The curves of Figure 1 of the drawings are time-temperature curves contrasting our new annealing practice as shown by line 2 with the old full annealing continuous practice, as exemplified by line 1. FiguresZ and 3 are two curves plotted from a large number of test results showing the most effective annealing and shelving temperatures for obtaining a maximum Rockwell B value of 55 in low carbon steel strip. In obtaining the data for Figure 2, all samples were held at the annealing temperatures indicated for 30 seconds, following which they were quenched to a shelf temperature of about 900 F. and held at such temperature for 30 seconds. The data for Figure 3 was obtained by annealing all samples at 1250 F. for 30 seconds following by quenching to and holding at shelf-temperatures indicated for 30 seconds.

These curves show that the annealing temperature must be between about 1250 and 1300 F., preferably about 1275 F., if maximum softness is to be attained. The strip need only be held at such temperature for about 30 seconds to insure temperature equalization.

Following this the strip should be cooled as quickly as possible and in less than 2 minutes to a temperature within the range of 800 to 1000 F. and held within this range for at least 30 seconds. It will be noted from Figure 3 that maximum softness is achieved when the strip is quenched to and held within the range of 850 to 925 F. After holding within this range for the indi-- cated time, the strip may be cooled as rapidly as desired to room. temperature or, if it is to be coated by a hotdipping operation, cooled to a desired intermediate temperature and introduced directly into the coating pot. If the heat treating step of the present invention is conducted in-line with a galvanizing operation, we find it preferable to quench to and hold the strip in the range of about 875 to 900 F., which temperature range is ideally suited for introducing it into the pot of zinc spelter.

The effectiveness of the new annealing practice is readily apparent from the following data that shows a.

We have discovered that a particular sub- 3;; comparison of production records before and after the new annealing practice was used.

The new practice permits meeting a maximum Rockwell B'specification of '55about 100% of the time, Whereas such consistency is'not attainable'by' the old practice below a value of Rockwell 58B? Of equal commercial significance is the fact the above improvements were obtained without the necessityof reducing .the strip speed through the annealing equipment which has the effect of increasing the production capacity thereof about 25%.

While we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illus-' tration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A method of continuously annealing low-carbon, cold-rolled ferrous strip comprising continuously passing the strip through an enclosure having heating, cooling and holding zones; equalizing the temperature of said strip in the heating'zone in'the range of 1250 to 1300 F. to recrystallize the cold-rolled structure there of, quickly cooling the temperature thereof to below 1000 F. in the cooling zone and then holding said strip for at least 30 seconds in the temperature range of 800 to 1000" F. in the holding zone of said enclosure.

2. A method of continuously annealing low-carbon, cold-rolled ferrous strip comprising continuously passing the strip through an enclosure havingheating, cooling and holding zones, equalizingthe temperature of said strip in the heating zone in the range of 1250 to 1300 F. to recrystallize the cold-rolled structure thereof, quickly cooling the temperature thereof in less than 2 minutes to below 1000 F. in the cooling zone and then holding said strip for at least 30 seconds in the temperature range of 800 to 1000 F. in the holding zone of said enclosure.

3. A method of continuously galvanizing cleaned coldrolled, low-carbon ferrous strip comprising continuously passing the cleaned strip through an enclosure in a nonoxidizing atmosphere, said enclosure having heating, cooling and holdingzones, equalizing the temperature thereof in the holding zone in the range of 1250 to 1300 F. to recrystallize the cold-rolled structure, quickly cooling to a temperature in the rangeof 850 to 925 F. in the cooling zone in less than 2 minutes, holding the temperature of said strip in said range for at least 30 seconds and then without exposure to oxidizinginfluence introducing said strip into a molten metal coating pot.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF CONTINUOUSLY ANNEALING LOW-CARBON, COLD-ROLLED FERROUS STRIP COMPRISING CONTINUOUSLY PASSING THE STRIP THROUGH AN ENCLOSURE HAVING HEATING, COOLING AND HOLDING ZONES, EQUALIZING THE TEMPERATURE OF SAID STRIP IN THE HEATING ZONE IN THE RANGE OF
 1250. TO 1300*F. TO RECRYSTALLIZE THE COLD-ROLLED STRUCTURE THEREOF, QUICKLY COOLING THE TEMPERATURE THEREOF TO BELOW 1000*F. IN THE COOLING THE TEMPERATURE THEREOF TO BELOW FOR AT LEAST 30 SECONDS IN THE TEMPERATURE RANGE OF 800 TO 1000*F. IN THE HOLDING ZONE OF SAID ENCLOSURE. 